The art of surfactants, particularly surfactants with fluorochemical chains, shows a preference for such surfactants with longer fluorochemical chains, e.g., C6-C10 in U.S. Pat. No. 2,803,615 and C6-C12 in U.S. Pat. No. 3,787,351. Even in hydrocarbon surfactants, increasing the chain length of the hydrocarbon chain decreases the CMC (critical micelle concentration); the decrease in CMC is roughly one order of magnitude for each —CH2CH2— added to the chain (Amphoteric Surfactants, edited by Eric G. Lomax, Marcel Dekker Inc. (1996), p. 13). The same trend has been noted in surfactants derived from the perfluorocarboxylic acids and sulfonic acids (Organofluorine Chemicals and their Industrial Applications, edited by R. E. Banks, Ellis Horwood Ltd. (1979), p. 56; J. O. Hendrichs, Ind. Eng Chem, 45, 1953, p. 103; M. K. Bernett and W. A. Zisman, J. Phys. Chem., 63, 1959, p. 1912). Data exist that suggest that only after the seven outermost carbon atoms are fully fluorinated do the contact angles of various liquids on the surface approach those of a perfluoro fatty acid monolayer (N. O. Brace, J. Org. Chem., 27, 1962, p. 4491; W. A. Zisman, Advan. Chem. 1964, p. 22). Since models to explain the actions of surfactants often invoke the monolayer of the surfactant on the air/liquid interface, one would expect the same to be true of fluorinated surfactants, and that activity of the surfactant is closely tied to its chain length.
We have found the same trend when the surface tensions of C4F9SO3K and C8F17SO3K were measured in water. The results are shown in FIG. 1. We have also found that the same trend can be observed in a series of homologous fluorinated glycamide salts shown in FIG. 2. In each case the surface tension of the resulting solution of the C4F9SO2N-containing surfactant was significantly higher at the same concentration than the C8F17SO2N-containing surfactant. Based on the above, use of a C4F9SO2N-containing surfactant in an application would be disadvantageous over a C8F17SO2N-containing surfactant because higher levels would be required and the end result would potentially be higher expense and adverse effects on the properties of the composition used in the application.